Patent GB-796825-A (GB)
Title: Improvements in or relating to casting devices for optical elements
IPC: B29D11/00
Filed: 1955-10-04
Abstract: 796,825. Molding lenses. ABBEY, A. (American Optical Co.). Oct. 4, 1955, No. 28278/55. Class 87(2). A device for use in casting optical elements comprises a supporting member 10 having an upper annular end shelf 14 and a vertically spaced aligned lower annular shelf 13, and a pair of separate mould halves, the lower mould half 15 having its lower surface 17, engaging the lower shelf 13, formed spherical in shape and the upper mould half 22 having its under surface 23, engaging the upper end shelf 14, also spherical, the upper surface portion 19 of the lower mould half and the central portion of the lower surface 23 of the upper mould half being controlled in shape in accordance with optical properties desired of the resultant cast optical elements and the annular shelves 13, 14 being spaced to control the spacing between the surfaces 19, 23 and lying in respective planes disposed with respect to one another in accordance with the thickness and optical properties desired in the cast elements. The transparent synthetic resin from which the lens is to be cast, e.g., polycyclohexyl methacrylate, polystyrene or polymethyl methacrylate, is placed between the mould halves, either by first putting it on the lower half 15 or by injecting it between the two halves through an opening provided in the supporting member. The resin is polymerized by heat, ultra-violet light or chemical means. A suitable atmosphere for the polymerization may be achieved by admitting a gas, e.g., CO 2 , through the injection opening or through openings in the lower part of the member 10 and ports leading to the material. The concave, spherical, lower and upper surfaces 17, 19 of the lower half 15, which is preferably of glass, are concentric about a point 21 and the convex spherical lower surface 23 of the upper half 22 has a radius 24 from a point 25, the points 21, 25 being on the co-axis 20 of the annular shelves 13, 14, on the latter of which the surface 23 rests. The edges of the shelves 13, 14 may be discontinuous, providing a series of supporting points. The surface 26, shown concave in Fig. 3, may be of any desired shape. Thus the optical axis will remain constant in spite of relative movement between the mould halves 15, 22, and their spacing, and therefore the thickness of the lens produced, depends on the depth of the bore 12. The power of the lens is determined by the radii of curvature of the surfaces 17, 19 and 23. The bore 11 may have its axis inclined to the axis of the bore 12 and the plane of the shelf 13 may be normal to the inclined axis of the bore 11, with the radius of the spherical surface 17 of the lower mould half having its centre on this inclined axis. The angle between the axes can be controlled to introduce the desired amount of prism in a lens. Alternatively the inclination of the axis of the bore 12 may be altered. The supporting member 10 may be in two part threaded to screw in and out of one another to adjust the distance between the shelves 13 and 14 for lenses of different thickness. The material shrinks during polymerization but as the mould halves are free to move it will not pull away from their surfaces and form imperfections in the lens. The mould halves are spaced to allow for the shrinkage.